1. Field of the Invention
The present invention relates in general to exhaust emission control systems for internal combustion engines and particularly to an internal combustion engine exhaust emission control system of the type having a catalytic converter and a wide-range A/F (air/fuel) ratio sensor disposed downstream of the catalyic converter.
2. Description of the Related Art
For cleaning the exhaust gas emitted from an automotive internal combustion engine, a catalyst carrying thereon precious metals as platinum and rhodium or other metals has heretofore been used. Such a catalyst oxidizes and reduces harmful pollutants as HC, CO, NOx in the exhaust gas and thereby cleans the exhaust gas. For effective cleaning, it is necessary to control an A/F ratio of the internal combustion engine. Particularly, for removing HC, OD, NOx from the exhaust gas simultaneously, it is necessary to control the A/F ratio so that the A/F ratio is accurately held at the stoichiometric ratio. To this end, as is well known, a sensor for detecting the A/F ratio is disposed upstream or downstream of the catalytic converter to control the A/F ratio of the mixture to be supplied to the internal combustion engine and therefore the oxygen (O.sub.2) concentration in the exhaust gas on the basis of the output of the sensor.
An oxygen sensor which is put into one of two alternative operation conditions in response to the oxygen concentration in the exhaust gas, is relatively widely used. However, in recent years, a wide-range A/F ratio sensor which is capable of detecting the value of the A/F ratio of itself is also widely used for the necessity of performing a feedback control of the A/F ratio within a lean A/F ratio region.
Referring to FIG. 1 which illustrates a principle of a wide-range A/F ratio sensor, the sensor includes two cells 21 and 22 each having a zirconia base and a pair of electrodes disposed on the opposite sides of the base. Formed between the two cells 21 and 22 is a measurement gap 24 into which the exhaust gas is introduced from an exhaust passage. On the side of the cell 22 opposite to the m assent gap 24, there is also formed an atmospheric chamber 25 into which the air is introduced to serve as a reference gas. Indicated by 26 is a heater and by 27 is an amplifier. In this A/F ratio sensor, the exhaust gas is conducted into the measurement gap 24 by diffusion. An electronic circuit regulates the current applied to the cell 21 to maintain a constant gas composition in the measurement gap 24. In case the A/F ratio is lean, the cell 21 is energized so that the oxygen in the exhaust gas and of nitrogen monoxide having come into the measurement gap 24 is pumped out of the measurement gap 24. On the basis of the current (pumping current) necessitated for this pumping, the A/F ratio is obtained. On the contrary, in case the A/F ratio is rich, the A/F ratio is obtained on the basis of the current (pumping current) necessitated for generating oxygen for oxidizing reducing substances as carbon monoxide, hydrogen and hydrocarbon within the measurement gap 24. As shown in FIG. 3, if the A/F ratio is stoichiometric, the pumping current is "0". Positive pumping current is produced if the A/F ratio is lean, and negative pumping current if rich.
In this maimer, basically, the concentration of oxides and the concentration of reducing substances existing in the exhaust gas are obtained on the basis of electric current. The exhaust gas enters the measurement gap 24 by diffusion, so there is a problem that depending upon the different diffusion speeds, the above described gaseous elements differ in the time necessary for them to arrive the detecting portion (i.e., electrode surface), from each other. The diffusion speed is almost dependent upon the size of molecule (molecule weight). Oxygen and nitrogen oxide which are the components to be detected w the A/F ratio is lean, are nearly equal in the molecule weight and therefore nearly equal in the diffusion speed, so that there is not caused any particular problem. However, hydrogen (H.sub.2)and carbon monoxide (CO) which are components to be detected when the A/F ratio is rich, differ largely in the molecule weight and therefore in the diffusion speed. Hydrogen which is smaller in the size of molecule diffuses faster than oxygen and therefore arrives the electrode surface faster. For this reason, in case the concentration of hydrogen and the concentration of carbon monoxide in the exhaust gas are the same, hydrogen arrives the electrode surface faster so hydrogen requires, for its oxidation at the electrode surface, four times the pumping current which is required by oxygen. That is, even if hydrogen and carbon monoxide are equal in the concentration, hydrogen which arrives the detecting portion faster is detected as being four times richer in the concentration of reduced gas. However, on the side upstream of the catalytic converter, the ratio between hydrogen and carbon monoxide contained in the exhaust gas is nearly constant at any time. Thus, by previously determining the relation between the pumping current and the A/F ratio when the exhaust gas is mixed with hydrogen and carbon monoxide at a constant rate, i.e., the characteristic indicated by the solid line in FIG. 3, the pumping current can be converted to the A/F ratio by using this characteristic when the actual A/F ratio is to be detected.
Though in FIG. 3 the relation between the pumping current and the A/F ratio is shown by the straight line, the both does not actually have such a simply proportional relation. Thus, When the pumping current is to be actually converted to the A/F ratio, the A/F ratio is read on a map storing values of A/F ratios corresponding to a plurality of values of pumping current and by interpolation thereof.